In general, the coordination of the operation of a plurality of independent operating units requires communication of a command to each unit which indicates to each unit what operation is to be performed, and a response from each unit which indicates the result of the operation performed. In synchronous systems, the latter can be achieved by simply providing no response if the operation was successful and some response if the operation encountered an exception condition or the like. However, in asynchronous systems, some mechanism must be provided to distinguish the successful response from the state which exists during the performance of the operation. In some systems, a polling technique has been employed to query each unit, either in parallel or in series, to determine the operating condition or result of each, while in other systems distinct signals are provided during the performance of the operation and upon successful completion of the operation. The former technique requires significant time and hardware/software to implement, while the latter often requires additional dedicated signal paths/states.